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THE SUN.

sciences. The discoveries which have recently been effectedin chemistry and physics by means of the spectroscope ; in-vestigations as to the simple or compound nature of substanceshitherto classed as elementary ; refined researches into thephenomena of the highest obtainable vacua; the most advancedmethods of photography; and some of the most importantproblems of meteorology, magnetism, and electricity, are allfound to be intimately connected with the study of the Sun ’s,physical condition.

But before we discuss other points of greater fascination,and in some respects of deeper interest, it may be well, by wayof preface, to explain what has been accomplished by meansof astronomical observations, and by other scientific methods,towards an accurate determination of the most fundamentalportion of all our knowledge of the Sun , viz., its distance fromthe Earth . For our greatest hindrance in all investigationsof its condition is the vastness of that distance ; and theimportance of its more accurate determination has of late beenbrought into special prominence in connection with the transitsof Venus of December 8th, 1874, and December 6th, 1882 ; thelatter of which will happily (weather permitting) be partlyvisible at Greenwich.

To find how far the earth is from the Sun is a problem wellworth solving, for almost every other astronomical measure-ment depends upon it. But it is in no wise an easy problem.It gives scope for the most refined investigations, and yet itis one in which, we believe, even those who have -had verylittle scientific or mathematical learning may take an intelli-gent interest. Its direct solution by the ordinary methodsused in a terrestrial survey is altogether impossible. A sur-veyor measures the distauce between two stations, a and b, asin Fig. I., and observes with his theodolite the directions inwhich a distant object, s, is seen from them. He is then, ingeneral, able, by means of certain trigonometrical formulae, tocalculate the distance of that object from either of his stations.But, in order to use his formulae with success, it is essentialthat the triangle formed by lines joining the stations with eachother and with the distant object, be what is termed a well-